This disclosure relates to methods of servicing a wellbore. More specifically, it relates to methods of treating a wellbore to remove filter cakes.
Natural resources such as gas, oil, and water residing in a subterranean formation or zone are usually recovered by drilling a wellbore down to the subterranean formation while circulating a drilling fluid in the wellbore. After terminating the circulation of the drilling fluid, a string of pipe, e.g., casing, is run in the wellbore. The drilling fluid is then usually circulated downward through the interior of the pipe and upward through the annulus, which is located between the exterior of the pipe and the walls of the wellbore. Next, primary cementing is typically performed whereby a cement slurry is placed in the annulus and permitted to set into a hard mass (i.e., sheath) to thereby attach the string of pipe to the walls of the wellbore and seal the annulus. Subsequent secondary cementing operations may also be performed.
During the drilling operation, a drilling fluid, also referred to as drilling mud, is conventionally circulated through the wellbore as the borehole is drilled. The presence of such a drilling mud aids in the drilling operation, for example, by removing cuttings from the wellbore, (e.g., by suspending and releasing cuttings returned to the surface), controlling pressures within the subterranean formation, sealing permeable and/or semi-permeable portions of the subterranean formation, stabilizing the subterranean formation, cooling and lubricating the drilling apparatus, and facilitating completion operations. As the drilling mud is circulated through the wellbore during a drilling operation, the drilling mud is deposited on the surfaces of the growing wellbore in a thin, low-permeability layer known as a filter cake, thus inhibiting the loss of mud and/or mud filtrate or other wellbore fluids to the formation during the drilling and/or other servicing operations. Prior to production, the filter cake is removed to allow the unimpeded flow of natural resources to the wellbore.
An ongoing challenge is to both efficiently remove the filter cake while minimizing damage to the formation or the wellbore. When strong acids, such as hydrochloric acid, or low pH solutions are used for the removal of the filter cake, there is a corrosion risk to wellbore equipment, (e.g., safety valves, casings, production tubing, mandrels, pipes, separators, pumps, etc.). The use of strong acids for filter cake removal can also lead to the formation of “wormholes,” which occurs when the strong acid attacks the filter cake in localized spots and creates holes in the filter cake, as opposed to removing the entire filter cake. One way to circumvent this problem would be by using high pH solutions (e.g., pH greater than 7) for the removal of filter cakes. However, the filter cake breaking agents that chelate the metal ions from the filter cake at low pH values are usually ineffective in high pH environments. Thus, an ongoing need exists for more effective compositions and methods of removing filter cakes in subterranean formations.